1. Field of the Invention
The present invention relates to a visual sensor using light receiving elements and a reference light beam such as laser beam in combination.
2. Description of the Related Art
In capturing an image of an object having a broad range of brightness gradation, it is necessary to adjust density of the captured image in conformity with a dynamic range of the visual sensor including an image capturing device such as a camera. There have been proposed several methods of adjusting the density of the captured image within the dynamic range of the visual sensor.
There is known form Japanese Patent Publications Nos. 10-21378 and 11-66279, a technique utilizing a method of an uniform ratio transformation in which density of each pixel of one of a plurality of images of an object captured in different conditions is multiplied by a predetermined ratio to be approximately adjusted to density of a corresponding pixel in another image, and a pixel not saturated in one image is substituted for a corresponding pixel saturated in the other image, to expand a dynamic range of the visual sensor.
The method disclosed in these Publications requires the density transformation of each pixel for adjusting densities of all pixels in a plurality of images, and comparison and substitution processing of the densities for obtaining a composite image. In the density transformation, processing of multiplication and division based on a density ratio between the corresponding pixels in the plurality of images is required to burden a heavy load of arithmetic processing to a processor.
Another method is known in which brightness of an incident light on each pixel is subjected to a logarithm transformation to determine density of each pixel in a view of processing for obtaining an edge image by differences between densities of adjacent pixels. A dynamic range of brightness of the incident light is compressed in this transformation. According to the logarithm transformation of brightness, a definition of an edge image based on the densities determined by the logarithm transformation depends on a ratio between degrees of the brightness of incident lights according to an equation log(I1)−log(I2)=log(I1/I2), where I1 and I2 represent degrees of the brightness of incident lights on adjacent pixels. Thus, this method solves a problem of dependency of the definition of an edge image upon an intensity of lighting and a reflectance of an object, i.e., the definition of an edge image in a dark area in the captured image is low and the definition of an edge image in a bright area in the captured image is high to cause imbalance of image.
Japanese Patent Publication No. 5-314253 describes the logarithm transformation of brightness in which a linear proportional transformation is applied in an area having a low brightness instead of the logarithm transformation in order to improve efficiency of usage of an image memory in the logarithm transformation of brightness. In this method, processing for the logarithm transformation of brightness is necessary for each pixel, to produce a considerable amount of arithmetic operation load to a processor.
Japanese Patent Publication No. 9-311927 discloses a technique of reducing halation by extending a low brightness range (increasing density) and compressing a high brightness range (decreasing density) for respective pixels using a brightness transformation table. This method has an advantage in reducing arithmetic operation load but it is necessary to prepare brightness transformation tables for various applications and thus is not advantageous in general purpose. Further, it is difficult to find out theory in preparing the transformation table and thus is prepared based solely on experience.
In detecting position/posture of an object using a visual sensor, an intensity of light incident on the visual sensor varies in dependence upon a brightness of an object which changes in accordance with ambient such as intensity of lighting or sunshine and a deviation of placement of an object (relative position/posture between the object and the visual sensor). Therefore, there is a case of causing halation in an area necessary for detection or including useful information can not be obtained because of darkness of the image. It has been desired to expand a dynamic range of the visual sensor with respect to brightness of the object.